SDHAF2 and breast cancer: While many studies identify the SdhAF2/SdhE/Sdh5 assembly factor as necessary for covalent flavinylation of Complex II5,9–11, Crisper-CAS9 ΔsdhAF2 breast cancer cells12 and ΔsdhE E. coli4 each can synthesize Complex II enzymes that retain covalent flavin, albeit at a reduced level.